Sometimes, when at a park, picnicking, or whatever, you want to listen to the radio in your vehicle. But if you leave the driver side door open, you get the constant pinging because the key is still in the ignition. If you turn the key to the "on" position, it will ping 5 more times and then stop. (And you need to turn off the fan because it will run in that position.)

There are a couple of dash warning lights that stay on in this position, though. One indicates engine mantenance is needed. I think the other one is the oil warning.

Is it harmful to leave the key position on but the engine not running? I would think not, but I don't know what the electronics are doing in that case.

 

It is not harmful to leave the key in ON without the engine running, but you do run the risk of draining your battery a lot faster than you would if you left it in ACC. This is because a lot more electrical components receive power with the key in ON than ACC, like the fuel injection system, ignition coil, etc. It gets somewhat annoying especially with later models like my '08, where the radio turns off after a set period of time if the key is left in ACC. My in-laws had this problem with their Saturn VUE at a drive-in movie, so they thought they'd leave the key in ON so the radio wouldn't shut off. By the time the movie ended I needed to jump start their car.

If you want to listen to the radio while outside the car, why not just roll down the driver's window, or open the rear sliding door? I'm pretty sure neither of those triggers the "key in ignition" alarm.

 

Both conclusions for KEY ON above are right: no, you wil not damage anything, and yes, you risk needing a boost start. However, just for the sake of technicalities, neither the coils nor the fuel injectors will starve the battery: yes the + side of the 12V is applied to all of these, but until the other side of coils and injectors is grounded, none of them will draw an ounce of current from the battery: they must be grounded by the "drivers" that are controlled by the computer and unless the computer sees engine rotation, it will not fire any of them. The computer WILL be active though, waiting for all sensors to tell what is happening, and even if the heater fan is set to the lowest position, it still runs at very low speed, ensuring a minimal amount of air circulation as mandated by safety considerations to avoid lethal carbon monoxide concentrations..